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Moon’s odd bulge may betray a turbulent history

By Hazel Muir

The ancient Moon might have moved on an elliptical orbit that would have made it grow and shrink as viewed from Earth

(Image: NASA/JPL)

The shape of the Moon might encode subtle clues about its orbit billions of years ago, say planetary scientists in the US. Their work has turned up one possible solution to a lunar mystery first identified in revolutionary France, two centuries ago.

Astronomers think the Moon formed when a Mars-sized body – about half the diameter of Earth – walloped into our planet about 4.5 billion years ago, spraying molten debris into space. The debris quickly clumped together to form the Moon at a distance of about 4 Earth radii from the Earth’s centre, and solidified 100 million to 200 million years later.

Over billions of years, the Moon gradually spiralled outwards to its current, near-circular orbit at about 60 Earth radii. Tidal forces (gravitational effects) from the Earth have made the Moon’s spin period equal to its orbital period, so one side permanently faces the Earth.

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Imagine instead that someone magically created the Moon today by placing a perfectly uniform ball of rock in the Moon’s current position. Immediately, it would no longer be uniform. The Earth’s tidal forces would stretch the barren satellite from the near to the far side, for instance, while the forces of the Moon’s rotation would make it bulge out a little around its equator. That means its gravity along different axes would be different.

Unexpected bulge

However, measurements by lunar satellites since the 1960s have shown that the Moon’s gravity variations do not arise solely from current stresses and strains. In particular, the Moon’s gravity is slightly stronger than expected on the axis pointing to Earth, as though there is an unexpected bulge in the middle of the far side.

Amazingly, French mathematician Pierre Simon de Laplace figured this out as early as 1799 by analysing subtle wobbles in the Moon’s spin and orbit. “It’s pretty scary how clever that was,” says Ian Garrick-Bethell from MIT in Cambridge, Massachusetts, US.

People have suggested several possible explanations. For instance, maybe a “fossil bulge” froze into the Moon when it solidified, preserving a record of the extreme stresses on it at that time.

To test this possibility, Garrick-Bethell and his colleagues solved equations for the Moon’s motion to find out if any particular ancient orbits and spin rates could account for the Moon’s mass distribution today.

Glimpse of the far side

Sure enough, the team found several solutions. Garrick-Bethell favours one in particular&colon; a highly elongated ancient lunar orbit, with the Moon, like the planet Mercury, spinning 1.5 times per orbit. “We’ve seen in Mercury that this is a plausible and stable orbit,” he says.

That would make the Moon especially dramatic viewed from the lifeless Earth of 4.3 billion years ago. Previous models suggested the Moon loomed huge in the sky and zipped through its phases in less than 24 hours – as opposed to the month or so of today.

But the new research implies it would have also grown and shrunk as it swung nearer and farther from the Earth, and revealed the far side we can no longer see. “It would have been cool to see the other side of the Moon,” Garrick-Bethell told New Scientist.

He adds that although the result is not conclusive, the work is a good starting point for narrowing down the possible histories of the Moon&colon; “We’re working on that and I think it would be great if other people did too.”